1. Field of the Invention
The present invention relates to a method of enhancing a radiation image, particularly a radiation image recorded on intensifying screens by exposure to readiation, particularly X-rays. This process is particularly useful in non-destructive testing such as industrial x-rays, diagnostic imaging for medical purposes, and the like. The process is used with phosphors that store energy from incident radiation and release a different type or wavelength of radiation when stimulated. The process of the present invention enhances the image released upon stimulation of the excited phosphors.
2. Description of the Prior Art
Recorded images are a spatial, normally planar, representations of either spatially or temporally variable original signals. A large proportion of such recordings, such as copies of documents and pictures, represent a one to one relationship with an original document or scene, frequently with magnification or reduction involved. Radiographic film images in medicine represent a class of images where the original is not visible to the human eye and must be formed by a combination of invisible radiation (e.g. x-rays) and a suitable transducer (fluorescent screen).
In all image forming systems, degradation of the original information occurs which normally manifests itself in two forms--(1) blurring of edges (reduced resolution, lower sharpness) and (2) random irregularities (noise, fog). In normal photographic images it has long been known that edge sharpness can be enhanced and noise reduced by masking the original with a negative unsharp mask of suitable contrast (usually lower than that of the original). Early work by J. A. C. Yule is representative of this photographic masking approach (U.S. Pat. Nos. 2,407,211, 2,420,636, 2,455,849) and more complex approaches are represented by Blearson et al in U.S. Pat. No. 3,615,433. An early attempt to use a raster scan of the image while measuring the instantaneous light values photoelectrically and attenuating the beam according to a predetermined relationship with the light value is disclosed by Folse in U.S. Pat. No. 3,011,395. The rapid development of the Space Program lead to the emergence of high efficiency digital means of analyzing, reconstituting and enhancing images. Median filtering as a means of enhancing edge contrast has also been studied (e.g. B. R. Frieden JOSA 66. 280-283 (1976)). In the medical radiography field this stimulated the development of computerized tomography and the digital processing of radiographs in general (S. R. Amety et al, SPIE 207, 210-211 (1979), and C. R. Wilson et al, SPIE 314, 327-330 (1981)). In these approaches the image has been divided into a large number of "pixels" by scanning. A moving window consisting of n.times.m pixels centered on pixel i with image value D.sub.i is investigated by an on line computer as pixels i are scanned. The arithmetic average D of the pixels within the window is then used to modify the central pixel value D.sub.i to a filtered value D'.sub.i by the algorithm EQU D'.sub.i =aD.sub.i -bD
The parameters a and b are chosen to give specific image characteristics but are constant over the scan of a single image.
The concept of varying parameters similar to a and b throughout the scan of the image based on certain local properties of the image has been studied and these patents (H. Kato et al U.S. Pat. Nos. 4,315,318 and 4,317,179 and M. Ishida et al U.S. Pat. No. 4,346,409) have disclosed particular relationships between the parameters and the values of D.sub.i or D which can give further image enhancement. These techniques do not however distinguish between noise and image edges as far as enhancement is concerned, and the higher the density D.sub.i or D the greater the enhancement. In other technology areas similar approaches have been made. Thus in E. Alparslau and F. Ince IEEE Vol SMC-11, 376-384 (1981) images are treated with an edge enhancement algorithm based in part on an adaptive parameter based on the difference between the maximum and minimum pixel values in the window at any point. In U.S. Pat. No. 4,237,481 final image data for printing plate production is treated by electronic circuits according to algorithms which combine sharp and unsharp image data with pixel parameters. U.S. Pat. No. 4,334,244 treats video signal images electronically according to algorithms based on the fixed average and wherein values acting on the instantaneous gradient of the image signal, the degree of edge enhancement being partly controlled by the dynamic noise of the system.